Methods for stabilizing a bioprosthetic tissue by chemical modification of antigenic carbohydrates

ABSTRACT

Methods are provided herein for modifying antigenic carbohydrate epitopes within a xenographic bioprosthetic tissue by oxidation of vicinal diols to form aldehydes or acids and subsequence reductive amination of aldehydes to form stable secondary amines, or amidation or esterification of acids to form stable amides or esters. Advantageously, methods provided herein mitigate the antigenicity of the bioprosthetic tissue while leaving the overall tissue structure substantially undisturbed, and thereby enhance the durability, safety and performance of the bioprosthetic implant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/163,557, filed Jun. 17, 2011, which claims the benefit of U.S. PatentApplication No. 61/355,948, filed Jun. 17, 2010, the entire disclosuresof which are incorporated by reference.

FIELD

Methods are provided herein relating to the field of bioprostheticimplants, and more particularly to the treatment of bioprosthetictissues to decrease post-implantation antigenicity and calcification inhost subjects.

BACKGROUND

A primary limitation of bioprosthetic implants made from animal tissuesis the occurrence of hyperacute rejection reactions in transplantrecipients. Such reactions are driven largely by the presence ofantigenic carbohydrate epitopes within implanted tissues, the mostcommon of which is the α-GAL glycoprotein epitope: D Galactose (α 1-3)Galactose (β 1-4) N acetyl Glucosamine-R-motif (α-GAL) is found onvascular endothelial tissues of all species with the exception of oldworld monkeys, great apes, and humans. The presence of α-GAL onharvested animal donor tissues elicits an immediate and powerful immuneresponse after transplantation into humans that can quickly destroysurrounding tissues and/or organs. The rapidity of the rejectionresponse is due to very high levels of preformed anti-α-GAL antibodiesin human subjects (nearly 1% of all antibodies in human blood areanti-α-GAL antibodies). The high levels of anti-α-GAL are an adaptiveresponse to the ubiquitous presence of bacteria bearing α-GAL epitopesin the human digestive tract.

The most common tissue sources for xenographic bioprosthetic tissues areequine (horse), ovine (sheep), porcine (pig) and bovine (cow) tissues,all of which bear α-GAL epitopes and are potentially antigenic. Oneapproach for limiting the antigenicity of bioprosthetic tissues is tochemically modify antigenic epitopes so that they are no longerrecognized by host antibodies. This is typically accomplished bychemical fixation, which involves exposing a bioprosthetic tissue to afixative agent (or tanning agent) that forms cross-linkages within(intramolecular cross-linkages) and/or between (intermolecularcross-linkages) polypeptides of the tissue. Examples of fixative agentsused for treating bioprosthetic tissues include formaldehyde,glutaraldehyde, dialdehyde starch, hexamethylene diisocyanate andpolyepoxy compounds. Glutaraldehyde is the most widely used fixativeagent and glutaraldehyde treatment is currently the standard approachfor stabilizing clinically useful bioprosthetic tissues. Examples ofglutaraldehyde fixed bioprosthetic heart valves include theCarpentier-Edwards® Stented Porcine Bioprosthesis, theCarpentier-Edwards® PERIMOUNT® Pericardial Bioprosthesis, and theEdwards PRIMA Plus™ Stentless Aortic Bioprosthesis, all available fromEdwards Lifesciences, Irvine, Calif. 92614.

Although chemical fixation can considerably limit the antigenicity ofbioprosthetic tissues, fixed tissues, particularly glutaraldehyde-fixedtissue, suffer from several drawbacks. For example, the protectiveeffects of glutaraldehyde fixation tend to deteriorate over the lifespanof bioprosthetic implants due to the labile Schiff Base cross-links,resulting in increased immunogenicity and impaired long-term stabilityand performance. In addition, glutaraldehyde treatment rendersbioprosthetic tissues more susceptible to calcification, particularlywhen an implant remains in place for an extended period of time (e.g.,more than ten years) due to their high levels of residual aldehydegroups. Structural valve deterioration (SVD) is the most common causefor early valve explantation, with tissue calcification the leadingcause of failure in bioprosthetic implants. These glutaraldehyde-derivedaldehydes are associated with high levels of calcium mineralization.

U.S. Pat. No. 6,861,211 to Levy and Vyavahare describes methods ofstabilizing a bioprosthetic tissue through chemical cross-linkingaffected by treating the tissue with an agent, such as periodate, thatoxidizes carbohydrate moieties of glycosaminoglycans (GAG) to generatealdehydes, and then treating the tissue with a bifunctional agent thatreacts with the carbohydrate aldehydes as well as reactive groups onadjacent proteins to cross-link the GAG to the surrounding tissuematrix. Like conventional glutaraldehyde fixation, the methods result inresidual reactive aldehyde groups, which serve as potential calciumbinding sites and thus destabilize the tissue by ultimately compromisingthe biomechanical properties of the material.

U.S. Pat. No. 6,383,732 (Stone) describes an alternative to chemicalmodification for limiting the antigenicity of bioprosthetic tissuesusing the enzyme alpha-galactosiadase to destroy α-GAL epitopes.Enzymatic approaches suffer from the general high cost of enzymepreparations and the fact that the large size of alpha-galactosiadaseand other enzymes prevents these protein structures from penetratingdeeply into tissues, such as the extracellular matrix of pericardialbioprosthetic tissues. Thus, enzyme-based treatments do not eliminateall of the epitopes targeted by an enzyme, particularly in the interiorof a bioprosthetic implant. In addition, alpha-galactosiadase and otherenzymes are specific for particular epitopes (e.g., α-GAL in the case ofalpha galactosiadase), making it highly difficult to limit theantigenicity of tissues containing multiple and/or unknown epitopes. Theenzymatic removal of cellular components and tissue structures can alsodegrade the biomechanical properties of the tissue. Moreover, theseenzyme treatments cannot be used with glutaraldehyde-fixed tissue sincethe enzyme's protein structure will react with the residual aldehydesand become covalently bound to the material. The result is an increasein foreign proteins and further degradation in tissue performance.

Accordingly, there remains a need in the art for the development of newand improved methods for reducing antigenicity and limitingcalcification of xenographic tissues, thereby enhancing the durability,stability, and performance of the tissues. These enhancedcharacteristics are consistent with the demands of bioprosthetic tissuesin vivo, including maintaining the structural, mechanical, andbiocompatible properties of, for example, heart valves.

BRIEF SUMMARY

Methods are provided herein for improving the stability, durability,and/or performance of a xenographic bioprosthetic tissue implant bychemically modifying antigenic carbohydrates within the bioprosthetictissue.

In some aspects, the methods comprise the steps of: treating thebioprosthetic tissue with an oxidizing agent which oxidizes vicinal diolmoieties of antigenic carbohydrates to form aldehydes or acids andtreating the bioprosthetic tissue with a capping agent, the cappingagent comprising a primary amine or alcohol which combines with thealdehydes or acids to form imines, amides or esters.

In some aspects, the methods comprise the steps of: treating thebioprosthetic tissue with a capping agent, the capping agent comprisinga primary amine or alcohol which combines with aldehydes or acids toform imines, amides or esters, and treating the bioprosthetic tissuewith a stabilizing agent, the stabilizing agent converting the imines tosecondary amines or the esters to amides.

In some aspects, the methods comprise the steps of: treating thebioprosthetic tissue with an oxidizing agent which oxidizes vicinal diolmoieties of antigenic carbohydrates to form aldehydes or acids; treatingthe bioprosthetic tissue with a capping agent, the capping agentcomprising a primary amine or alcohol which combines with the aldehydesor acids to form imines, amides or esters; and treating thebioprosthetic tissue with a stabilizing agent, the stabilizing agentconverting the imines to secondary amines or the esters to amides.

In some aspects, the antigenic carbohydrate is N-glycolylneuraminic acid(Neu5Gc) in some aspects the antigenic carbohydrate is the Forssmanantigen (GalNAc alpha1,3GalNAc beta1,3Gal alpha1,4Gal beta1,4Glc-Cer).In further aspects, the antigenic carbohydrate comprises an α-galactosyl(α-Gal) epitope.

In some aspects, the oxidizing agent is a periodate. In some aspects,the periodate selectively oxidizes vicinal diols of antigeniccarbohydrates relative to β-aminoalcohol and/or vicinal diketone groupscomprising the bioprosthetic tissue.

In some aspects, the capping agent is a primary amine. In furtheraspects, the primary amine reacts with aldehydes on the bioprosthetictissue to form imines.

In some aspects, the capping agent is an alcohol. In further aspects,the alcohol reacts with acids on the bioprosthetic tissue to formesters.

In some aspects, the stabilizing agent is a reducing agent. In furtheraspects, the reducing agent converts bioprosthetic tissue imines tosecondary amines and esters to amides.

In some aspects, the bioprosthetic tissue is treated with the oxidizingagent in the presence of the capping agent. In further aspects, thebioprosthetic tissue is washed sufficiently to remove the oxidizingagent prior to treatment with the reducing agent.

In some aspects, the bioprosthetic tissue is treated with thestabilizing agent in the presence of the primary amine or alcoholcapping agent. In further aspects, the bioprosthetic tissue is treatedwith the capping agent and the stabilizing agent concurrently. In yetfurther aspects, the bioprosthetic tissue is washed to remove theoxidizing agent prior to treatment with the capping agent and/or thestabilizing agent.

In some aspects, the bioprosthetic tissue has been treated with one ormore of a surfactant and/or a fixative agent. In various aspects, thefixative agent is selected from the group consisting of an aldehyde, adialdehyde, a polyaldehyde, a diisocyanate, a carbodiimide, aphotooxidation agent, and a polyepoxy compound and the surfactant isselected from the group consisting of an anionic surfactant, an alkylsulfonic acid salt, a polyoxyethylene ether, a pluronic or tetronicsurfactant, and an alkylated phenoxypolyethoxy alcohol.

In some preferred aspects, the bioprosthetic tissue has been treatedwith glutaraldehyde.

In some preferred aspects, the bioprosthetic implant is a heart valve.In further aspects, the bioprosthetic tissue is bovine pericardium orporcine aortic valve. In yet further aspects, the bioprosthetic implantis a pediatric heart valve.

In some aspects, the oxidized bioprosthetic tissue is substantiallynon-immunogenic in a human host. In further aspects, the antigeniccarbohydrate of the treated bioprosthetic tissue is substantiallynon-antigenic in a human host. In yet further aspects, the treatedbioprosthetic tissue is substantially non-calcifying in a human host. Insome aspects, the human host is a pediatric patient.

In some aspects, the oxidizing agent is a periodate. In further aspects,the periodate is sodium periodate. In some aspects, the sodium periodateis used at a concentration of 20 mM. In some aspects, the bioprosthetictissue is treated with sodium periodate for about 3 hours at about 25°C.

In some aspects, the method further comprises treating the bioprosthetictissue with one or more of a surfactant and a fixative agent. In furtheraspects, the method comprises treating the bioprosthetic tissue with analdehyde fixative agent. In yet further aspects, the method comprisestreating the bioprosthetic tissue with glutaraldehyde. In some aspects,the fixative agent is carbodiimide (such as EDC). In some aspects, thefixative agent is diepoxy.

In some aspects, the bioprosthetic tissue is a fresh tissue.

In some aspects, the method further includes treating the bioprosthetictissue with a bioburden reduction solution including formaldehyde,ethanol, and a Tween® solution. In some aspects, the method furtherincludes drying the bioprosthetic tissue with ethanol and glycerol. Insome aspects, the method further includes sterilizing the bioprosthetictissue with ethylene oxide.

In some aspects, the method further includes decellularizing thebioprosthetic tissue with a decellularization method including treatingthe tissue with 0.1% SDS, rinsing the tissue, and treating the tissuewith DNAse. In some aspects, the method further includes drying andelectrophoretically cleaning the bioprosthetic tissue. In some aspects,the method further includes sterilizing the bioprosthetic tissue withglutaraldehyde.

In some aspects, the method further includes treating the bioprosthetictissue with a bioburden reduction solution comprising ethanol and aTween® solution.

Other aspects are described in co-owned U.S. Pub. No. 2009/0164005,filed Dec. 18, 2008, herein incorporated by reference in its entirety,for all purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows various aspects of the tissue treatment process as providedin the present disclosure. As shown in FIG. 1, the process generallyincludes vicinal diol (i.e., vic Diol) oxidation, treatment with acapping agent, and/or treatment with a stabilizing agent.

FIGS. 2A-2B show immunohistochemistry for α-Gal expression followingtreatments of un-fixed tissues. Blue is DNA (DAPI staining) and Green isα-Gal (Isolectin IB4 staining). FIG. 2A shows minimal staining in fresh,un-fixed tissue treated with the process described herein (vic Dioloxidation, capping agent, and reducing agent). FIG. 2B shows a lighterarea of intense α-Gal (Isolectin IB4 staining) in fresh, un-fixed tissuetreated with periodate only.

FIGS. 3A-3C show immunohistochemistry for α-Gal expression on un-fixedtissues treated with various types of periodate. Stained areas are shownas lighter areas compared to the darker background. Blue is DNA (DAPIstaining) and Green is α-Gal (Isolectin IB4 staining). FIG. 3A showsun-fixed tissue treated with a 1% SDS/DNAse decellularization procedure.FIG. 3B shows un-fixed tissue treated with a commercially availabledecellularized collagen tissue. FIG. 3C shows un-fixed tissue treatedwith another commercially available decellularized collagen tissue.

FIGS. 4A-4F show that tissue fixed with glutaraldehyde has severeautofluorescence, with FIGS. 4A-4C depicting tissue stained withisolectin dye and FIGS. 4D-4F depicting unstained tissue.

FIGS. 5A-5B show α-Gal and DNA expression as darker areas on fixedtissue treated with ThermaFix (TFX) only. Brown is α-Gal (Isolectin-IB4,DAB) and Blue is nuclei (Hemotoxylin staining). FIG. 5C is aflow-diagram of the process used for this experiment.

FIGS. 6A-6D show α-Gal and DNA expression as darker areas on fixedtissue treated with the combined treatment of fixed tissue with TFX andthe process described herein. FIGS. 6A-6B show tissue treated withethanolamine. FIGS. 6C-6D show tissue treated with taurine. Brown isα-Gal (Isolectin-IB4, DAB) and Blue is nuclei (Hematoxylin staining).

FIGS. 7A-7B show α-Gal and DNA expression as darker areas on fixedtissue treated with a capping, reduction, and drying process. FIG. 7C isa flow-diagram of the process used for this experiment, Brown is α-Gal(Isolectin-IB4, DAB) and Blue is nuclei (Hematoxylin staining).

FIGS. 8A-8D show α-Gal and DNA expression as darker areas on fixedtissue treated with the combined treatment [e] with TFX and vicinal diol(i.e., vic Diol) oxidation, treatment with a capping agent, treatmentwith a reducing/stabilizing agent, and drying as described herein. FIGS.8A-8B show tissue treated with ethanolamine. FIGS. 8C-8D show tissuetreated with taurine. Brown is α-Gal (Isolectin-IB4, DAB) and Blue isnuclei (Hematoxylin staining).

FIG. 9 shows the percent of total isolectin-B4-α-Gal binding inhibitionas induced by various tissue treatments and as compared to a control.

FIG. 10 shows the anti-α-Gal response for the following treatments:glutaraldehyde; TFX; vic Diol oxidation, treatment with a capping agent,and treatment with a stabilizing agent, and a capping, reduction anddrying treatment in primate subjects. Data is given as a percentincrease or decrease in absorbance compared to an original value.

DETAILED DESCRIPTION

Descriptions of the invention are presented herein for purposes ofdescribing various aspects, and are not intended to be exhaustive orlimiting, as the scope of the invention will be limited only by theappended claims. Persons skilled in the relevant art can appreciate thatmany modifications and variations are possible in light of the aspectteachings.

Unless defined otherwise, all technical and scientific terms used hereinhave the meaning commonly understood by one of ordinary skill in theart. While exemplary methods and materials are described herein, it isunderstood that methods and materials similar or equivalent to thosedescribed can be used. All publications mentioned herein areincorporated by reference to disclose and describe the methods and/ormaterials in connection with which they are cited.

It must be noted that, as used in the specification, the singular forms“a”, “an” and “the” include plural referents unless the context clearlydictates otherwise.

Methods are provided herein for mitigating the immunogenicity of axenographic bioprosthetic tissue by chemically modifying one or moreantigenic carbohydrates within the tissue while leaving the overalltissue structure substantially unmodified.

In some aspects, the methods comprise the steps of: treating thebioprosthetic tissue with an oxidizing agent which oxidizes vicinal diolmoieties of antigenic carbohydrates to form aldehydes or acids andtreating the bioprosthetic tissue with a capping agent, the cappingagent comprising a primary amine or alcohol which combines with thealdehydes or acids to form imines, amides or esters.

In some aspects, the methods comprise the steps of: treating thebioprosthetic tissue with a capping agent, the capping agent comprisinga primary amine or alcohol which combines with aldehydes or acids toform imines or esters, and treating the bioprosthetic tissue with astabilizing agent, the stabilizing agent converting the imines tosecondary amines or the esters to amides.

In some aspects, the methods comprise the steps of: treating thebioprosthetic tissue with an oxidizing agent which oxidizes vicinal diolmoieties of antigenic carbohydrates to form aldehydes or acids; treatingthe bioprosthetic tissue with a capping agent, the capping agentcomprising a primary amine or alcohol which combines with the aldehydesor acids to form imines or esters; and treating the bioprosthetic tissuewith a stabilizing agent, the stabilizing agent converting the imines tosecondary amines or the esters to amides.

Without being limited by a particular theory, it is believed thatglutaraldehyde fixation and other established methods for stabilizingxenographic tissues suffer from several limitations that are associatedwith antigenicity, calcification, and long-term failure of bioprostheticimplants. Glutaraldehyde and other fixative agents stabilize tissues byforming cross-linkages between certain reactive moieties within thetissues without necessarily altering or eliminating antigenic epitopes.Glutaraldehyde fixation reduces antigenicity in a largely indirectmanner due to the adsorption of host immune cells, antibodies, and serumproteins to concentrated aldehyde groups on the surfaces ofglutaraldehyde fixed tissues, forming a coating of native molecules thatisolates the tissue from host immune factors. However, such proteincoatings deteriorate over time, exposing the tissue to the host immunesystem. In addition, the interior of glutaraldehyde fixed tissues oftencontains high levels of “latent antigens” due to the slow rate ofpenetration and diffusion of glutaraldehyde throughout treated tissues.As a result, glutaraldehyde fixed bioprosthetic tissues can becomeincreasingly antigenic over time, leading to calcification, tissuefatigue, and eventually failure of the bioprosthetic implant.

Advantageously, methods provided herein reduce the antigenicity and/orcalcification of bioprosthetic tissues by addressing one or morelimitations associated with glutaraldehyde fixation and/or otherestablished methods. Treating bioprosthetic tissues with periodateaccording to the instant methods selectively oxidizes antigeniccarbohydrates, resulting in the covalent modification of xenographicantigens. In addition, periodate and other chemical agents are smallmolecules that readily diffuse throughout bioprosthetic tissues,including chemically fixed tissues, to eliminate latent antigensthroughout the tissues. Methods provided herein use a capping agent toconvert aldehyde groups produced by periodate oxidation and/orglutaraldehyde fixation to imines, and a reducing agent to convert thehydrolytically unstable imines to stable and substantially non-antigenicsecondary amines. The methods thus eliminate reactive and toxicaldehydes and prevent further oxidation of aldehydes to acids that serveas potential calcium-binding sites. Moreover, calcification is furtherreduced by the modification of latent antigens and the resultingdecreased immunogenicity of bioprosthetic tissues. Advantageously,methods provided herein improve the stability, durability, and/orperformance of bioprosthetic tissue implants.

In some aspects, the “antigenic carbohydrate” targeted for modificationby the instant methods is a glycosaminoglycan (GAG) polysaccharide thatis found on glycoproteins and/or glycolipids of a xenographic tissue andis recognized as foreign by the immune system of a human subject.Antigenic carbohydrates within bioprosthetic tissues can trigger varyinglevels of immune responses that can decrease the performance,durability, and/or lifespan of the implant and potentially requireimmediate medical intervention to replace the implant. In some aspects,antigenic carbohydrates modified according to methods provided hereinare “periodate labile” in that they comprise one or more exposed vicinaldiol (R¹—CH(OH)CH(OH)—R²) moieties capable of being oxidized by aperiodate to produce a pendant aldehyde (R¹CHO). Advantageously,periodate oxidation of an antigenic carbohydrate modifies its structureso that it is no longer recognized by circulating antibodies. In somepreferred aspects, treating a glutaraldehyde fixed tissue with periodateaccording to a method provided herein substantially eliminates periodatelabile antigenic carbohydrate epitopes. In further aspects, treating aglutaraldehyde fixed tissue with periodate according to a methodprovided herein renders the tissue substantially non-antigenic.

In some aspects, an antigenic carbohydrate modified according to theinstant methods is the α-GAL epitope (Galα₁₋₃Galβ₁₋₄GlcNAc-R). Treatingan α-GAL-expressing xenographic tissue with periodate according to themethods provided herein results in oxidation of the vicinal diol of theα-GAL terminal galactose, producing two pendant aldehydes. The pendantaldehydes are preferably converted to imines by a primaryamine-containing “capping agent”, and the imines are converted to stablesecondary amines by a reducing agent. Advantageously, periodateoxidation of the terminal galactose unit modifies the α-GAL epitope suchthat it is no longer recognized by human anti-α-GAL (“anti-GAL”)antibodies, thus substantially reducing the antigenicity of thebioprosthetic tissue.

In further aspects, an antigenic carbohydrate modified according to theinstant methods is the sialic acid N-glycolylneuraminic acid (Neu5Gc),the so called Hanganutziu-Deicher (HD) antigen, which comprises anine-carbon sugar with a periodate labile vicinal diol. Neu5Gc is commonin mammalian tissues, especially porcine tissues, but is not synthesizedendogenously by humans. Nevertheless, Neu5Gc is sometimes detected atrelatively stable levels in humans due to dietary intake and possiblemetabolic incorporation of small amounts of Neu5Gc in humanglycoproteins. Human subjects have varying levels of circulatingantibodies against Neu5Gc, with the highest levels comparable to thoseof anti-GAL antibodies. Advantageously, periodate oxidation of Neu5Gcsialic acid residues within a xenographic tissue modifies the Neu5Gcepitope so that it is no longer antigenic to human subjects.

In further aspects, the antigenic carbohydrate is the Forssman antigen(GalNAc alpha1,3GalNAc beta1,3Gal alpha1,4Gal beta1,4Glc-Cer). M.Ezzelarab, et al. Immunology and Cell Biology 83, 396-404 (2005).

In some preferred aspects, treating a bioprosthetic tissue according toa method provided herein significantly reduces the antigenicity of thetissue in a human subject. In further aspects, treating a bioprosthetictissue according to a method provided herein renders the tissuesubstantially non-antigenic in a human subject. In yet further aspects,methods provided herein significantly reduce antigenicity and/or renderthe tissue substantially non-antigenic in a human pediatric subject.

In some aspects, bioprosthetic tissues treated according to the instantmethods have been treated with a fixative agent. As used herein, theterms “fixed” or “fixation” refer generally to the process of treatingbiological tissue with a chemical agent (a fixative agent) that formsintermolecular and intramolecular cross-linkages within and betweenstructures in order to stabilize the tissue structure and preventdegradation. For example, fixation reduces the susceptibility of tissuesto proteolytic cleavage by preventing the unfolding and denaturationrequired for proteases to access potential substrate proteins.Glutaraldehyde, formaldehyde, dialdehyde starch, and other aldehydecross-linking agents are the most commonly used fixative agents fortreating bioprosthetic tissues in preparation for surgical implantation.While fixation with fixative agents is desirable for stabilizing thetissue, fixation can also generate reactive chemical moieties in thetissue that are capable of binding calcium, phosphate, immunogenicfactors, or other precursors to calcification. For example,glutaraldehyde fixation produces a high concentration of free aldehydeswhich are intrinsically toxic and can be further oxidized to formnegatively charged carboxylic acid groups that serve as potentialbinding sites for positively charged calcium ions.

The term “calcification” as used herein, means deposition of one or morecalcium compounds, such as calcium phosphate, calcium hydroxyapatite,and/or calcium carbonate, within a bioprosthetic tissue, which can leadto undesirable stiffening and/or degradation of the bioprosthesis.Although the precise mechanisms underlying calcification are unclear,calcification is generally known to arise in bioprosthetic tissues outof the interaction of plasma calcium ions with free aldehydes,phospholipids, and other tissue components. In addition, bioprosthetictissues are particularly prone to calcification in pediatric subjects.Calcification can be intrinsic or extrinsic with respect to abioprosthetic tissue. Intrinsic calcification is characterized by theprecipitation of calcium and phosphate ions at sites within abioprosthetic tissue, such as the extracellular matrix and remnantcells. Extrinsic calcification is characterized by the precipitation ofcalcium and phosphate ions on external sites on a bioprosthetic tissueby, e.g., thrombus formation or the development of surface plaques.Advantageously, methods provided herein reduce both intrinsic andextrinsic forms of calcification.

In some preferred aspects, treating a bioprosthetic tissue according toa method provided herein significantly reduces the level ofcalcification in the tissue and/or the propensity of the tissue forcalcification in a human subject. In further preferred aspects, treatinga bioprosthetic tissue according to a method provided herein renders thetissue substantially non-calcifying in a human subject. In yet furtheraspects, methods provided herein significantly reduce the level ofand/or the propensity for calcification of a tissue and/or render atissue substantially non-calcifying in a human pediatric subject.

The effects of the instant methods on reducing and/or eliminatingxenographic antigens, free aldehydes, and/or calcification (or thepropensity for calcification) can be detected using a variety of methodsknown to those skilled in the art. The mitigation of antigeniccarbohydrates can be monitored by, e.g., direct galactose assays (α-GALepitopes), immunohistochemical staining (e.g., using anti-α-GAL and/oranti-Neu5Gc antibodies), and conventional histology. The level of freealdehydes in a tissue can be measured spectrophotometrically using acolorimetric reagent, such as4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (available under thetradename PURPALD), which reacts specifically with aldehydes to yieldcolored 6-mercapto-triazolo-(4,3-b)-s-tetrazines detectable at 550 nm,as described, e.g., in Dickinson and Jacobsen, Chem. Commun., 1719(1970). A reduction in the concentration of free aldehydes in abioprosthetic tissue can also be measured as a reduction in the toxicityof the tissue. For example, a bioprosthetic tissue (or sample thereof)can be used as a substrate for seeding cultured endothelial cells, andthe growth of the endothelial cell monolayer on the bioprosthetic tissuesubstrate can provide a sensitive biological indicator of the number andconcentration of residual aldehydes in the tissue.

The extent of calcification of a bioprosthetic tissue can be measuredusing a variety of methods known in the art, such as spectrophotometricmethods (e.g., as described by Mirzaie et al., Ann. Thorac. Cardiovasc.Surg., 13:2 (2007)) and spectroscopic methods (e.g., inductively-coupledplasma mass spectroscopy (ICP-MS) after nitric acid ashing).Calcification of tissues may also be assayed by histological staining(e.g., Von Kossa staining) or by using a calcification indicator (e.g.,eriochrome black T, murexide, or o-cresolphthalein, as described, e.g.,in Sarkar et al., Anal. Biochem., 20:155-166 (1967)). In addition,calcification of heart valve bioprosthetic implants can be detected byassociated changes in the mechanical properties of the tissue, such asincreased stiffening, which can be detected visually and/or measuredusing various methods known in the art. Those skilled in the art will befamiliar with these and other methods.

As used herein, the term “bioprosthetic” refers to any prosthesis whichis implanted in a mammalian subject, preferably a human subject, andderived in whole or in part from animal or other organic tissue(s).Bioprosthetic implants used in methods provided herein include tissue“patches,” heart valves and other heart components, heart replacements,vascular replacements or grafts, urinary tract and bladder replacements,bowel and tissue resections, and the like.

Bioprosthetic implants treated according to methods provided herein canbe derived from any biological tissue, including but not limited to,heart valves, blood vessels, skin, dura mater, pericardium, cartilage,ligaments and tendons. In some aspects, the tissue used to prepare abioprosthetic implant is selected according to the degree of pliabilityor rigidity, which varies with the relative amounts of collagen andelastin present within the tissue, the structure and conformation of thetissue's connective tissue framework (e.g., arrangement of collagen andelastin fibers), and/or other factors known to those skilled in the art.Bioprosthetic tissues having relatively high levels of collagen, such asheart valve tissue and pericardial tissue, have been found to beparticularly suitable for human bioprosthetic heart valve implant.However, those skilled in the art will realize that the instant methodscan be used to treat bioprosthetic implants made from any suitabletissue.

In some preferred aspects, the bioprosthetic implant is a heart valveimplant that is derived from a xenographic mammalian donor tissue andintended for use in a human subject. In further preferred aspects, thebioprosthetic implant is derived from a xenographic mammalian donorother than a great ape or an old world monkey, such as but not limitedto, an equine donor, an ovine donor, a porcine donor or a bovine donor.

Those skilled in the art will recognize that the instant methods areparticularly beneficial in treating those prostheses for whichpost-implantation degeneration and/or calcification poses a significanta clinical problem. For example, in some aspects, the bioprostheticimplant is a heart valve formed from bovine pericardium or porcineaortic valve and designed for implantation in a human subject. In yetfurther preferred aspects, the bioprosthetic implant is derived from axenographic mammalian donor tissue and is designed for implantation in ahuman pediatric subject.

An “oxidizing agent” according to the present methods includes any mildoxidizing agent that is suitable for the selective oxidation ofantigenic carbohydrates having vicinal diols to produce free aldehyde oracid moieties. Oxidizing agents according to the present disclosure canbe halogen series oxidizing agents or peroxide series oxidizing agentsor the like. Examples of oxidizing agents include, but are not limitedto, periodic acid, salts of periodic acid such as sodium periodate, leadtetraaceatate, hydrogen peroxide, sodium chlorite, sodium hypochlorite,potassium permanganate, oxygen, halogens such as bromine and othersknown to those skilled in the art.

In some aspects, the oxidizing agent is a periodate. A “periodate”according to methods provided herein is a compound comprising aperiodate ion (IO₄ ⁻) that is capable of reacting, as shown in thereaction scheme below, with vicinal diol moieties (1) of antigeniccarbohydrates to yield two pendant aldehyde moieties (2) along withformic acid and H₂O.

In some aspects, oxidation of vicinal diols is carried out in an aqueoussolution, preferably an aqueous buffered solution, under conditionssuitable for maintaining the structure and biological properties of thebioprosthetic tissue. In some aspects, a periodate is used for oxidationof vicinal diols. Typically, a stoichiometric amount of periodate isused to oxidize vicinal diol moieties, which amount can be determinedempirically for a particular volume of tissue and/or for a particulartype of tissue. Alternatively, a stoichiometric excess or periodate canbe used. Solutions are generally buffered to have a pH between about 4and about 9, with a pH between about 6 and about 8 desired for certainpH sensitive biomolecules. Periodate oxidation is generally carried outat a temperature between about 0 and about 50 degrees Celsius, andpreferably at a temperature between about 4 and about 37 degreesCelsius. Depending on the antigenic carbohydrate(s) targeted formodification, the size and geometry of the bioprosthetic tissue and/orother considerations, periodate oxidation can be carried out for aperiod of between a few minutes to as long as many days. Preferably,periodate oxidation is carried out for a period between about severalhours and about 24 hours. Long-term oxidation reactions are preferablyperformed under conditions that prevent over-oxidation. Treatment timesand temperatures for periodate oxidation tend to be inversely related,in that higher treatment temperatures require relatively shortertreatment times. Those skilled in the art will recognize that theprecise reaction conditions for a particular bioprosthetic tissue can bedetermined by routine experimentation, using methods known in the art.

In various aspects, the oxidizing agent is capable of oxidizing vicinaldiols within antigenic carbohydrates targeted for modification, formingeither pendant aldehyde moieties, which are converted to imines and thento more stable secondary amines by methods provided herein, or acids,which are converted directly to amides, or alternatively, converted toesters and then to more stable amides by methods provided herein. Insome aspects, the size, charge, and/or other characteristics of theoxidizing agent allow it to readily penetrate and diffuse throughout thebioprosthetic tissue and be washed out of the tissue after a desiredduration of treatment. In some aspects, the oxidizing agent is aperiodate that is a periodic acid or a salt thereof, such as sodiumperiodate, potassium periodate, or another alkali metal periodate salt.In some preferred aspects, the oxidizing agent is sodium periodate. Insome aspects the oxidizing agent is an acetate, such as lead acetate.

In some aspects, treating a bioprosthetic tissue with a periodateaccording to a method provided herein results in selective oxidation ofvicinal diols relative to other reactive functionalities, including butnot limited to, 2-aminoalcohols (e.g., on N-terminal serine, N-terminalthreonine or 5-hydroxylysine residues), 1,2-aminothiols (e.g., onN-terminal cysteine residues), and vicinal diketones. In some preferredaspects, treating a bioprosthetic tissue with an oxidizing agentaccording to methods provided herein selectively oxidizes vicinal diolswithin one or more antigenic carbohydrates while leaving non-targetedstructures substantially unmodified.

Without being limited to a particular theory, it is believed thatpotentially reactive moieties within bioprosthetic tissues vary in theirsusceptibility to oxidation with the following general order ofreactivity (from most to least labile): vicinal diols, 2-aminoalcohols,1,2-aminothiols, and vicinal diketones. In addition, the selectivity ofan oxidizing agent for vicinal diols can be further enhanced by treatingtissues with the oxidizing agent under mildly oxidizing conditionsSkilled artisans will recognize that mildly oxidizing conditions can bedetermined empirically using various methods known in the art, such ascarrying out oxidation reactions under varying conditions with a mixtureof carbohydrate substrates and monitoring the rate of production ofreaction products. For example, the stringency of oxidation can bemodulated by adjusting various reaction conditions, such as oxidizingagent concentration, treatment duration, temperature, solutionchemistry, and the like.

In some aspects, a bioprosthetic tissue is treated with an oxidizingagent under conditions that favor oxidation of a particular antigeniccarbohydrate. For example, antigenic carbohydrates having a sialic acidterminal sugar, such as Neu5Gc, are generally more susceptible toperiodate oxidation than those having other terminal sugars, such asgalactose (e.g., α-GAL).

In some aspects, the oxidizing agent selectively oxidizes vicinal diolsof antigenic carbohydrates targeted for modification relative to otherpotentially labile moieties on biomolecules comprising the bioprosthetictissue.

A “capping agent” according to the present methods includes any cappingagent capable of reacting with free aldehyde or acid moieties. Thecapping agent can be a primary amine or an alcohol. In various aspects,the capping agent is R⁴-M-NH₂, wherein: R⁴ is H, C₁₋₆ alkyl, S(═O)₂OR⁵,C₁₋₆ alkoxy, or hydroxyl; M is a linker, wherein the linker is C₁₋₆alkylene; and R⁵ is H or C₁ alkyl. In further aspects, R⁴ is H. In yetfurther aspects, R⁴ is S(═O)₂OR⁵ and R⁵ is H. In certain aspects, thecapping agent is an amine, alkyl amine, hydroxyl amine, aminoether,amino sulfonate, or a combination thereof.

Examples of capping agents include, but are not limited to,ethanolamine; taurine; amino acids such as glycine and lysine; alkoxyalkyl amines, such as 2-methoxyethylamine; n-alkyl amines such asethylamine, and propylamine, N-Hydroxysuccinamide (NHS),N-Hydroxysulfosuccinamide (NHSS), and others known to those skilled inthe art.

Chemical moieties referred to as univalent chemical moieties (e.g.,alkyl, alkoxy, etc.) also encompass structurally permissible multivalentmoieties, as understood by those skilled in the art. For example, whilean “alkyl” moiety generally refers to a monovalent radical (e.g.,CH₃CH₂—), in appropriate circumstances an “alkyl” moiety can also referto a divalent radical (e.g., —CH₂CH₂—, which is equivalent to an“alkylene” group).

All atoms are understood to have their normal number of valences forbond formation (e.g., 4 for carbon, 3 for N, 2 for 0, and 2, 4, or 6 forS, depending on the atom's oxidation state). On occasion a moiety can bedefined, for example, as (A)_(a)B, wherein a is 0 or 1. In suchinstances, when a is 0 the moiety is B and when a is 1 the moiety is AB.

Where a substituent can vary in the number of atoms or groups of thesame kind (e.g., alkyl groups can be C₁, C₂, C₃, etc.), the number ofrepeated atoms or groups can be represented by a range (e.g., C₁-C₆alkyl) which includes each and every number in the range and any and allsub ranges. For example, C₁-C₃ alkyl includes C₁, C₂, C₃, C₁₋₂, C₁₋₃,and C₂₋₃ alkyl.

“Alkoxy” refers to an O-atom substituted by an alkyl group as definedherein, for example, methoxy (—OCH₃, a C₁alkoxy). The term “C₁₋₆ alkoxy”encompasses C₁ alkoxy, C₂ alkoxy, C₃ alkoxy, C₄ alkoxy, C₅ alkoxy, C₆alkoxy, and any sub-range thereof.

“Alkyl” refer to straight and branched chain aliphatic groups havingfrom 1 to 30 carbon atoms, or preferably from 1 to 15 carbon atoms, ormore preferably from 1 to 6 carbon atoms, each optionally substitutedwith one, two or three substituents depending on valency. “Alkyl”includes unsaturated hydrocarbons such as “alkenyl” and “alkynyl,” whichcomprise one or more double or triple bonds, respectively. The term“C₁₋₆ alkyl” encompasses C₁ alkyl, C₂ alkyl, C₃ alkyl, C₄ alkyl, C₅alkyl, C₆ alkyl, and any sub-range thereof. Examples of such groupsinclude, without limitation, methyl, ethyl, propyl, isopropyl, butyl,tert-butyl, isobutyl, pentyl, hexyl, vinyl, allyl, isobutenyl, ethynyl,and propynyl.

“Alkylene” refers to a divalent radical that is a branched or unbranchedhydrocarbon fragment containing the specified number of carbon atoms,and having two points of attachment. An example is propylene(—CH₂CH₂CH₂—, a C₃alkylene). The term “C₁₋₆ alkylene” encompasses C₁alkylene, C₂ alkylene, C₃ alkylene, C₄ alkylene, C₅ alkylene, C₆alkylene, and any sub-range thereof.

“Amine” refers to a —N(R*)R** group, wherein R and R′ are independentlyhydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl,heterocyclyl, or heteroaryl as defined herein. In the case of a primaryamine, R* and R** are each H.

A “substituted” moiety is a moiety in which one or more hydrogen atomshave been independently replaced with another chemical substituent. As anon limiting example, substituted phenyl groups include 2-fluorophenyl,3,4-dichlorophenyl, 3-chloro-4-fluorophenyl, and2-fluoro-3-propylphenyl. In some instances, a methylene group (—CH₂—) issubstituted with oxygen to form a carbonyl group (—CO).

An “optionally substituted” group can be substituted with from one tofour, or preferably from one to three, or more preferably one or twonon-hydrogen substituents. Examples of suitable substituents include,without limitation, alkyl, alkenyl, alkynyl, cycloalkyl,heterocycloalkyl, aryl, heteroaryl, aroyl, halo, hydroxy, oxo, nitro,alkoxy, amino, imino, azido, mercapto, acyl, carbamoyl, carboxy,carboxamido, amidino, guanidino, sulfonyl, sulfinyl, sulfonamido,formyl, cyano, and ureido groups.

Carboxylic acid groups like those in glutamic acid or gamma carboxyglutamic acid are known to bind calcium atoms. Calcium binding proteinssuch as bone sialoprotein contain carboxylic acid-rich domains designedto attract and bind calcium, leading to hydroxyapatite formation(calcification). The overall level and location of acid groups in theseproteins determines the ability of the protein to efficiently bindcalcium and form hydroxyapatite. The term “acid potential” of the tissuerefers to the level of these chemical functional groups within the fixedtissue which may eventually form acid groups or “binding sites” byoxidation, dehydration, hydration, or similar processes.

Calcium binding causes significant post-implant damage in bioprostheticmaterials, especially tissues used for heart valve leaflets. Forexample, the oxidative damage that occurs during storage and handling ofdehydrated or “dry” tissue can create carboxylic acid groups that willbind calcium and lead to tissue failure. This progressive leaflet damageprocess can create new binding sites or potential binding sites that areprecursors to calcification and immunogenic related pathways. Thepresent disclosure provides for a method for capping these newly formedbinding sites prior to implantation of the tissue for tissue-basedbioprosthetic into the body. Bioprosthetic tissue exposed to oxidationfrom the atmosphere when not submersed in a glutaraldehyde solution orduring sterilization is likely to contain more acid groups thatcontribute to calcification and inflammation. In dry storage, thedehydrated tissue is sterilized and stored “dry” without the protectiveeffect of the glutaraldehyde solution. The ease of handling and storageof this new product is greatly facilitated due to the absence of theglutaraldehyde storage solution. This technology can be improved bytreating such bioprosthetic tissue with a capping agent and/or adding achemical protectant during the dehydration phase.

As shown in the reaction scheme below, a “capping agent” according tomethods provided herein is in some aspects a primary amine(R′NH₂)-containing agent (3) capable of reacting with free aldehydes(R¹CHO) (2) to form imines (R³N═CHR¹) (4).

Aldehyde Capping (Schiff Base Reaction):

In some aspects, the capping reaction is carried out independently ofoxidation in a neutral or slightly basic solution, at a temperaturebetween about 0 and about 50 degrees Celsius, for a period of severalminutes to many hours. Preferably, the reaction is carried out at a pHbetween about 6 and about 10, at a temperature between about 4 and about37 degrees Celsius, and for a period of about 1 to about 3 hours. Thoseskilled in the art will recognize that the precise reaction conditionsfor a particular bioprosthetic tissue can be determined by routineexperimentation, using methods known in the art.

One chemical target within the invention is the permanent “capping” ofthe acid groups which dramatically reduces their ability to attractcalcium, phosphate, immunogenic factors, or other groups. The term“capping” refers to the blocking, removal, or alteration of a functionalgroup that would have an adverse effect on the bioprosthesis properties.For example, the addition of1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC),N-hydroxysulfosuccinimide (sulfo-NHS) and ethanolamine will effectivelycap the acid groups with a non-reactive esters.

Preferably, the capping agent is capable of reacting with aldehydes oracids produced by oxidation of vicinal diols and/or by chemical fixationwith an aldehyde fixative agent (e.g., glutaraldehyde) to form imines oresters under conditions suitable for maintaining the structure andfunction of the bioprosthetic implant. In various aspects, the cappingagent can be an amine, an alkyl amine (e.g., ethylamine orisopropylamine), a hydroxyl amine (e.g., ethanolamine), an aminoether(e.g., 2-methoxyethylamine), an amino sulfonate (e.g., taurine, aminosulfates, dextran sulfate, or chondroitin sulfate), an amino acid (e.g.,lysine or beta-alanine), a hydrophilic multifunctional polymer (e.g.,polyvinyl alcohol or polyethyleneimine), hydrophobic multifunctionalpolymer (α-dicarbonyls, methylglyoxal, 3-deoxyflucosone, or glyoxal), ahydrazine (e.g., adipic hydrazide), mono-, di- or polyepoxy alkanes, orcombinations thereof.

In some aspects, the capping agent is a monoamine. Without being limitedby a particular theory, it is believed that certain agents comprisingtwo or more primary amine groups can mediate cross-linking and othernon-specific reactions within the bioprosthetic tissue. In somepreferred aspects, the capping agent is selected from ethanaolamine,taurine (2-aminoethanesulfonic acid), 2-methoxyethylamine, andethylamine. Advantageously, using a monoamine capping agent convertsfree aldehydes within a bioprosthetic tissue into stable secondaryamines without forming residual reactive groups and/or altering thebasic structural and/or mechanical properties of the tissue.Advantageously, using an alcohol capping agent such as ethanolamine,acids produced by oxidation of vicinal diols can be converted intostable esters without forming residual reactive groups and/or alteringthe basic structural and/or mechanical properties of the tissue.

In some aspects, the capping reaction is performed concurrently withvicinal diol oxidation to prevent sequential oxidation of aldehydes tocarboxylic acids. The reaction can be carried out under essentiallysimilar conditions as described above for oxidation. In further aspects,the bioprosthetic tissue is washed to remove the oxidation agent priorto treatment with the reducing agent.

In some preferred aspects, the bioprosthetic tissue is pre-treated witha chemical fixative agent, such as glutaraldehyde. Fixation limitspotential cross-reactivity between aldehydes formed by oxidation andother reactive moieties within the tissue by extensively cross-linkingthe tissue and/or modifying reactive functionalities. For example,primary amines found on lysine and hydroxylysine residues of collagensand other proteins comprising the extracellular matrix can potentiallycompete with the capping agent in reactions with aldehydes formed byoxidation of vicinal diols and such competing reactions can have anegative impact on the structure and/or stability of the tissue.Chemical fixation with an aldehyde fixative agent, such asglutaraldehyde, substantially eliminates such competing reactions bycross-linking reactive amines within the tissue and stabilizing theoverall tissue structure.

In some aspects, a bioprosthetic tissue is pre-treated with a protectingagent that couples to reactive moieties within the tissue and preventsundesired cross-linking and/or other reactions. For example, lysineamino acid residues may be protected or blocked by a number of methodsknown in the art, including but not limited to, the use of tertbutyloxycarbonyl (Boc), benzyloxycarbonyl (Z),biphenylisopropyloxycarbonyl (Bpoc), triphenylmethyl (trityl),9-fluoroenylmethyloxycarbonyl (Fmoc) protecting groups. Protectinggroups may be preferred in cases where a bioprosthetic tissue isincompatible with chemical fixation, for example because of a need topreserve the native biological structure and/or activity of the tissue.

Advantageously, treating a fixed and/or oxidized bioprosthetic tissuewith a capping agent according to the instant methods eliminatespotential binding sites for calcium, phosphate, immune factors, and/orother undesirable factors. In further aspects, treating a bioprosthetictissue with a capping agent according to the instant methods replacesaldehydes and/or acids within the tissue with a chemical moiety thatimparts one or more beneficial properties to the tissue, such as areduction in local and/or overall net charge, improvedhemocompatibility, increased hydration, or improved mechanicalflexibility. For example, treating a bioprosthetic tissue with thecapping agent taurine replaces aldehydes with a sulfonate group whichcan be beneficial for tissue hydration, flexibility, and/orcompatibility with host tissues. Furthermore, treating a bioprosthetictissue with the capping agent ethanolamine replaces acids with estermoieties, thereby improving the biocompatibility of the tissue.

A “stabilizing agent” according to the present methods includes anychemical agent capable of reacting with free aldehyde or acid moieties.In various aspects, the stabilizing agents are reducing agents. Thestabilizing agents are selected from the group consisting of sodiumborohydride, sodium cyanoborohydride, lithium aluminum hydride, directatmospheric or high pressure hydrogenation, carbodiimides such as1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), pyridines such as2-chloro 1-methyl pyridinium iodide(CMPI) and similar Mukaiyama'scondensation reagents, and others known to those skilled in the art.

In some aspects, the present capping process can include chemicalreduction of the tissue, which, when applied to the tissue in thepresence of a capping agent, will permanently connect the capping agentto the target group. For example, the addition of ethanolamine to thetissue will cap the aldehyde groups, while the reducing agent (e.g.,sodium borohydride) reduces any Schiff base created by reaction of thealdehyde with the amine group of ethanolamine. Thus an aldehyde isultimately replaced by a stable chemical moiety, which may be beneficialfor tissue hydration, flexibility, and cell interactions. Of course,other capping agents can be used instead of ethanolamine and otherreducing agents other than sodium borohydride and are known by thoseskilled in the art and which are included in the scope of this patent.Another strategy provided by the present methods is to oxidize thetissue aldehydes to acids, and then cap the acid groups. This mayinvolve the addition of 1-ethyl-3-[3-dimethylaminopropyl]carbodiimidehydrochloride (EDC), N-hydroxysulfosuccinimide (sulfo-NHS), orethanolamine. These new “capped” groups will reduce the attraction ofcalcium, phosphate, immunogenic factors, or other similar agents.

In some aspects, the stabilizing agent is a reducing agent. A “reducingagent” according to methods provided herein is any agent capable ofconverting esters to amides or imines to secondary amines. In variousaspects, the stabilizing agent is a reducing agent and can convertimines to secondary amines as shown in the reaction scheme below. Asshown below, imines (4) produced by reaction of the capping agent withaldehydes (2) are reduced to form secondary amines (5) by using asuitable reducing agent.

Imine Reduction:

Imine reduction may be carried out under essentially the same conditionsdescribed above for the periodate oxidation and capping agent steps. Insome aspects, imine reduction is carried out in a neutral or slightlybasic solution, at a temperature between about 0 and about 50 degreesCelsius, and for a period of about a few minutes to many hours.Preferably, the pH is between about 6 and about 10, the temperature isbetween about 4 and about 37 degrees Celsius, and the reaction period isbetween about 3 to about 8 hours. In some aspects, the complete sequenceof reactions is complete within about 24 hours.

The reaction of an aldehyde moiety (R¹CHO) with the primary amine moiety(R³NH₂) of a capping agent produces a hemiaminal intermediate whichforms the imine in a reversible manner through the loss of H₂O. In someaspects, the bioprosthetic tissue is treated with the capping agentseparately from treatment with the reducing agent. The isolated iminereaction product is then converted to a secondary amine with a suitablereducing agent, such as but not limited to, sodium borohydride.

In some preferred aspects, the bioprosthetic tissue is treated with thereducing agent concurrently with the capping agent, such that imineformation and reduction of the hydrolytically unstable imine occurconcurrently to form a secondary amine. In some preferred aspects, thebioprosthetic tissue is treated concurrently with the capping agent anda reducing agent that is selective for imines relative to aldehydesand/or ketones, such as but not limited to, sodium cyanoborohydride(NaBH₃CN), sodium triacetoxyborohydride (NaBH(OCOCH₃)₃), or acombination thereof.

In some aspects, aldehydes produced by oxidation and/or chemicalfixation are reductively aminated directly, without formation of theintermediate imine, by treating a periodate oxidized bioprosthetictissue with a reducing agent in an aqueous environment, e.g., asdescribed in Dunsmore et al., J. Am. Chem. Soc., 128(7): 2224-2225(2006).

In a particular aspect, an oxidation/capping and stabilization scheme isused involving the treatment of the tissue with a periodic acid salt toselectively cleave the vicinal diols of the carbohydrates, followed bytreatment of the tissue with a secondary mild oxidizing agent such assodium chlorite or hydrogen peroxide to convert the aldehydes to acids;then capping the acids with a capping agent selected from the groupconsisting of N-hydroxysuccinamide and N-hydroxysulfosuccinamide to forman ester; and then stabilizing the cap by converting the ester to anamide by the action of a carbodiimide stabilizing agent such as1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC).

Also provided herein is a method for improving the performance of abioprosthetic implant, the method including: obtaining the bioprosthetictissue, wherein the bioprosthetic tissue is a fresh tissue;decellularizing the bioprosthetic tissue; fixing the bioprosthetictissue with a fixation agent comprising glutaraldehyde; exposing thetissue to an initial bioburden reduction solution; at least partiallyfabricating a bioprosthetic product or device; treating the at leastpartially fabricated bioprosthetic tissue product or device with asecond bioburden reduction solution comprising formaldehyde, ethanol,and a Tween® solution; treating the at least partially fabricatingbioprosthetic product or device with a periodate, wherein the tissueexpresses an antigenic carbohydrate including a vicinal diol, andwherein the vicinal diol is oxidized by the periodate to form analdehyde; treating the bioprosthetic tissue with a capping agent,wherein the capping agent comprises a primary amine, and wherein theprimary amine reacts with the aldehyde to form an imine; treating thebioprosthetic tissue with a reducing agent, wherein the reducing agentreacts with the imine to form a secondary amine; drying thebioprosthetic tissue; and sterilizing the at least partially fabricatedbioprosthetic product or device with ethylene oxide.

Also provided herein is a method for improving the performance of abioprosthetic implant, the method including: obtaining the bioprosthetictissue, wherein the bioprosthetic tissue is a fresh tissue;decellularizing the bioprosthetic tissue; fixing the bioprosthetictissue with a fixation agent including glutaraldehyde; exposing thetissue to an initial bioburden reduction solution, at least partiallyfabricating a bioprosthetic tissue product or device; treating the atleast partially fabricated bioprosthetic tissue product or device with asecond bioburden reduction solution including formaldehyde, ethanol, anda Tween® solution; treating the at least partially fabricatedbioprosthetic product or device with a periodate, the tissue expressingan antigenic carbohydrate including a vicinal diol, wherein the vicinaldiol is oxidized by the periodate to form an aldehyde; treating thebioprosthetic tissue with a capping agent, wherein the capping agentincludes a primary amine, wherein the primary amine interacts with thealdehyde to form an imine; treating the bioprosthetic tissue with areducing agent, wherein the reducing agent interacts with the imine toform a secondary amine; drying and electrophoretically cleaning thebioprosthetic tissue; and sterilizing the at least partially fabricatedbioprosthetic product or device with ethylene oxide.

Also provided herein is a method for improving the performance of abioprosthetic implant, the method including: obtaining the bioprosthetictissue, wherein the bioprosthetic tissue is a fresh tissue;decellularizing the bioprosthetic tissue; fixing the bioprosthetictissue with a fixation agent including glutaraldehyde; exposing thetissue to an initial bioburden reduction solution; at least partiallyfabricating a bioprosthetic product or device; treating the at leastpartially fabricated bioprosthetic tissue product or device with asecond bioburden reduction solution including formaldehyde, ethanol, anda Tween® solution; treating the at least partially fabricatedbioprosthetic product or device with a periodate, the tissue expressingan antigenic carbohydrate including a vicinal diol, wherein the vicinaldiol is oxidized by the periodate to form an aldehyde; treating thebioprosthetic tissue with a capping agent, wherein the capping agentincludes a primary amine, wherein the primary amine interacts with thealdehyde to form an imine; treating the bioprosthetic tissue with areducing agent, wherein the reducing agent interacts with the imine toform a secondary amine; drying and electrophoretically cleaning thebioprosthetic tissue; and sterilizing the at least partially fabricatedbioprosthetic product or device glutaraldehyde.

Also provided herein is a method for improving the performance of abioprosthetic implant, the method including: obtaining the bioprosthetictissue, wherein the bioprosthetic tissue is a fresh tissue;decellularizing the bioprosthetic tissue; fixing the bioprosthetictissue with a fixation agent including glutaraldehyde; exposing thetissue to an initial bioburden reduction solution; at least partiallyfabricating a bioprosthetic product or device; treating the at leastpartially fabricated bioprosthetic tissue product or device with asecond bioburden reduction solution including formaldehyde, ethanol, anda Tween® solution; treating the at least partially fabricatedbioprosthetic product or device with a periodate, the tissue expressingan antigenic carbohydrate including a vicinal diol, wherein the vicinaldiol is oxidized by the periodate to form an aldehyde; treating thebioprosthetic tissue with a capping agent, wherein the capping agentincludes a primary amine, wherein the primary amine interacts with thealdehyde to form an imine; treating the bioprosthetic tissue with areducing agent, wherein the reducing agent interacts with the imine toform a secondary amine; drying and electrophoretically cleaning thebioprosthetic tissue; and sterilizing the at least partially fabricatedbioprosthetic product or device drying and electrophoretically cleaningthe bioprosthetic tissue; and sterilizing the bioprosthetic tissue withethylene oxide.

In various aspects, bioprosthetic tissues subject to methods providedherein may be pre-treated with one or more secondary stabilizing agents,including but not limited to, a fixative agent and/or a skinning agent.

The instant methods are compatible with fresh, partially and fully fixedbioprosthetic tissues. Fixative agents useful for pre-treatingbioprosthetic tissues used in methods provided herein include, but arenot limited to, aldehydes (e.g., formaldehyde, glutaraldehyde,dialdehyde starch, acrolein, glyoxal acetaldehyde), polyglycidyl ethers(e.g., Denacol 810), diisocyanates (e.g., hexamethylene diisocyanate),carbodiimide(s), and epoxides (e.g., any of the various Denacols andtheir individual reactive species, including mono, di, tri, andmulti-functionalized epoxides). In some preferred aspects, thebioprosthetic tissue has been previously fixed with glutaraldehyde,which has proven to be relatively physiologically inert and suitable forfixing a variety of biological tissues for subsequent surgicalimplantation (Carpentier, A., J. Thorac. Cardiovasc. Surg. 58:467-68(1969)). An exemplary protocol for glutaraldehyde pre-treatment is setforth in Example 1. Fixation with glutaraldehyde or another fixativeagent can provide a variety of benefits, including increased stability,increased durability, improved preservation, increased resistance toproteolytic cleavage.

In some aspects, the bioprosthetic implant is a commercially availablebioprosthetic heart valve, such as the Carpentier-Edwards® stentedporcine bioprosthesis, Edwards Lifesciences, Irvine, Calif., theCarpentier-Edwards® Pericardial Bioprosthesis, Edwards Lifesciences,Irvine, Calif., or the Edwards® PRIMA Stentless Aortic Bioprosthesis,Edwards Lifesciences AG, Switzerland, which has been treated accordingto a method provided herein.

In further aspects, the bioprosthetic tissue is a fresh, non-fixedxenographic tissue harvested from a mammalian host, which is treatedaccording to methods provided herein and implanted into a host subject.

The tissue to be treated can be freshly harvested from an abattoir, itcan be washed and pre-treated with various decellurizing agents, and/orit can be at least partially fixed with fixative agents. After thestabilization step, the tissue can also be treated by decelluarizationmethods, various fixation methods, bioburden reduction, drying andglycerolization, and final sterilization steps. It is understood that ingeneral some or all of the sequential steps can be combined intosimultaneous steps e.g., the oxidation and capping step, the capping andstabilization steps, or all three steps can react in concert. Likewisesome or all of the pre- and post-carbohydrate antigen mitigation stepscan be combined into a smaller set of various simultaneous steps.

A number of surfactants may be used in accordance with the presentmethods, including but not limited to, anionic surfactants (e.g., estersof lauric acid, including but not limited to, sodium dodecyl sulfate),alkyl sulfonic acid salts (e.g., 1-decanesulfonic acid sodium salt),non-ionic surfactants (e.g., compounds based on the polyoxyethyleneether structures, including Triton X-100, 114, 405, and N-101 availablecommercially from Sigma Chemical, St. Louis, Mo., and relatedstructures, and pluronic and tetronic surfactants, availablecommercially from BASF Chemicals, Mount Olive, N.J.), alkylatedphenoxypolyethoxy alcohols (e.g., NP40, Nonidet P40, Igepal, CA630,hydrolyzed/functionalized animal and plant compounds including, Tween®80, Tween® 20, octyl-derivatives, octyl b-glucoside, octylb-thioglucopyranoside, deoxycholate and derivatives thereof,zwitterionic compounds, 3-([cholamidopropyl]-dimethylammonio-1-propanesulfonate (CHAPS), 3-([cholamidopropyl]-dimethylammonio)-2-hydroxy-1-propanesulfonate (CHAPSO)), and mixtures thereof(e.g., deoxycholate/Triton, Micro-80/90).

In some aspects a tissue is treated with a cell disrupting agent. Celldisrupting agents can include a hypotonic saline of 0% to 0.5% NaCl,non-ionic, anionic, and/or cationic detergents, and surfactants, e.g.,Tweens, sodium dodecyl sulfate (SDS), sodium deoxycholate, tetradecylammonium chloride, and benzalkonium chloride. In one aspect, CHAPSO inthe range of 0% to 5% can be used as a cell disrupting agent.

In some aspects a tissue is treated with a proteolytic inhibitorincluding, e.g., Protinin or EDTA.

In some aspects a tissue is treated with a lipid, phospholipid, cellmembrane, and/or cell remnant extracting agent. Such extracting agentscan include alcohols (e.g., ethanol, 2-propanol, or n-decanol in theconcentration range of 1% to 100%); ketones (e.g., acetone, methyl ethylketone); ethers (e.g., diethyl ether, tetrahydrofurane, 2-methoxyethanol); surfactants and detergents (e.g., Tweens®, sodium dodecylsulfate (SDS), sodium deoxycholate, tetradecyl ammonium chloride,benzalkonium chloride); CHAPSO; or Supercritical fluids (e.g., CO₂, NO).

In some aspects a tissue is treated with an anti-antigenic enzyme (e.g.,DNAse, RNAse).

In some aspects a tissue is treated with a bioburden reducing agent,including: antibiotics (e.g., penicillin, streptomycin); alcohols (e.g.,ethanol, 2-propanol, n-decanol in the concentration range of 1% to100%); aldehydes (e.g., formaldehyde, acetaldehyde, glutaraldhyde in therange of 0% to 5%).

In one aspect, a tissue is treated with a bioburden reducing solutionthat is a combination of formaldehyde, ethanol, and tween-80 (FETs) in aconcentration of about 1%/22.5%/0.1%, respectively.

In some aspects, a fabrication device is used for at least partiallyfabricating a bioprosthetic product or device. The fabrication devicecan be any device that is suitable for the assembly of a bioprostheticproduct or device.

Those skilled in the art will appreciate that various alternative agentssuitable for pre-treating bioprosthetic tissues are known in the art andmay be substituted for those indicated herein.

Having now generally described the invention, the same will be morereadily understood through reference to the following examples which areprovided by way of illustration, and are not intended to be limiting ofthe disclosed invention, unless specified.

Exemplary Aspects Example 1 Tissue Pre-Treatment

Prior to chemically modifying the antigenic carbohydrates in axenographic bioprosthetic tissue, the tissue may optionally bepre-treated by exposure to cross-linking agents and/or surfactants. Thefollowing non-limiting procedure sets forth one potential tissuepre-treatment protocol that produces fixed tissues. Those skilled in theart will appreciate that various alternative methods, chemicalcompounds, or solutions may be substituted for those indicated.

Step 1: Harvest/Prepare Biological Tissue

A desired biological tissue is harvested (surgically removed or cut awayfrom a host animal) at a slaughterhouse, placed on ice, and transportedto the location at which the bioprosthesis will be manufactured.Thereafter, the tissue is typically trimmed and washed with a suitablewashing solution, such as a saline solution, sterile water, or a basicsalt solution. For example, harvested tissues can be rinsed, washed,and/or stored in a phosphate or non-phosphate buffered saline solutionthat includes an organic buffering agent suitable for maintaining thesolutions at a physiologically compatible pH without deleterious effectsto the tissue. Both phosphate and non-phosphate buffering agents aresuitable for tissue processing. The following buffering agents, at aconcentration of about 10 mM to about 30 mM, are generally suitable fornon-phosphate buffered saline solutions used herein: acetate, borate,citrate, HEPES (N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid),BES (N, N-bis[2-hydroxyethyl]-2-amino-ethanesulfonic acid), TES(N-tris[hydroxymethyl]methyl-2-aminoethanesulfonic acid), MOPS(morpholine propanesulfonic acid), PIPES(piperazine-N,N′-bis[2-ethane-sulfonic acid]), or MES (2-morpholinoethanesulfonic acid). The buffering agent HEPES, which has a pKa ofabout 7.4, is well-suited for tissue processing. Advantageously, the useof a non-phosphate buffered organic saline solution typically decreasesthe likelihood of calcium precipitation on a bioprosthetic tissue.

Buffered saline solutions used in the instant methods may also comprisea chelating agent, which preferably binds divalent cations, such ascalcium, magnesium, zinc, and manganese. Examples of suitable chelatingagents include EDTA (ethylenediaminetetraacetic acid), EGTA(ethylenebis(oxyethylenenitrilo)tetraacetic acid),ethylenebis(oxyethylenenitrilo)tetraacetic acid, citric acid or saltsthereof, and sodium citrate, at concentrations of about 20 mM to about30 mM. Advantageously, the removal of divalent cations by the chelatingagent renders the tissue less susceptible to spontaneous precipitationof the divalent ions with phosphate ions that may be present in thetissue.

In one aspect, the non-phosphate buffered organic saline solution isisotonic and comprises about 0.9 wt-% saline, about 10 mM to about 30 mMHEPES buffer, pH 7.4, and about 20 mM to about 30 mM of EDTA.

Step 2: Glutaraldehyde Fixation of Biological Tissue

The harvested, trimmed and washed tissue is disposed within a containerfilled with a 0.625% solution of glutaraldehyde comprising approximately26 mL/L glutaraldehyde (25%); approximately 4.863 g/L HEPES buffer;approximately 2.65 g/L MgCl₂.6H₂O; and approximately 4.71 g/L NaCl. Thebalance of the solution comprises double filtered H₂O. NaOH is added toadjust the pH to approximately 7.4. The glutaraldehyde solution canoptionally contain a sterilant (e.g., 2% (w/w) ethanol) and/or askinning agent (e.g., 1% (w/w) Tween® 80). Where the glutaraldehydesolution contains a sterilant and/or skinning agent, the tissue isincubated at a controlled temperature (e.g., between about 20 to 37° C.)with continuous circulation of the solution for a period of betweenabout 2 to 24 hours, typically about 9 hours. The tissue is then washedand incubated in glutaraldehyde solution without sterilant or skinningagent at a controlled temperature (e.g., 50±5° C.) with continuouscirculation for a period of between about 7 to 14 days to completeglutaraldehyde fixation. Room air is allowed to blanket or cover theglutaraldehyde solution throughout the process. Glutaraldehyde fixedtissues prepared according to the instant methods are preferably fixedunder conditions that allow the tissues to be immersed in 6 Nhydrochloric acid at 110° C. for 5 days with minimal degradation.

Step 3: Assembly/Fabrication of Prosthesis

After completion of Steps 1 and 2, the tissue is rinsed with a suitablerinsing solution such as buffered saline or 0.625% glutaraldehyde.Thereafter, the tissue may be transported into a clean room or asepticenvironment, further trimmed or shaped (if necessary) and assembled withany non-biological components (e.g., stents, frames, suture rings,conduits, segments of polyester mesh to prevent suture tear-through,etc.) to form the desired implantable bioprosthetic device.

Example 2 Selective Chemical Modification of Antigenic Carbohydrates

Chemical modification of antigenic carbohydrates in a xenographicbioprosthetic tissue, as described herein, may be performed whether ornot the tissue is pre-treated. The following non-limiting procedure setsforth methods for chemically modifying select antigenic carbohydrates ineither scenario.

After the bioprosthetic tissue has been rinsed and stored, the tissue ispreferably immersed in isotonic buffered saline solution containing aperiodate oxidizing agent, such as sodium periodate, at a concentrationof about 20 mM for a period of about 20 minutes at room temperature withconstant agitation.

After treatment with the periodate oxidizing agent, the tissue is rinsedextensively in 20% ethanol to completely remove the periodate,preferably in a vessel allowing a large solution to tissue volume ratioto create a favorable gradient for solute diffusion.

The tissue is then immersed in a solution containing a primary aminecapping agent and a reducing agent suitable for converting any freealdehydes within the tissue to secondary amines. In one method, thetissue is immersed in isotonic buffered saline solution with a pH of 8.5containing a capping/reducing solution comprised of taurine andisoproplyamine 50%/50% 20 mM and 10 mM sodium borohyride at roomtemperature for a period of about 10 minutes with constant agitation.The tissue is then washed and treatment with the capping/reducingsolution is repeated for a total of three 10 minute treatments with thecapping/reducing solution.

The bioprosthetic tissue is removed from the capping/reducing agentsolution, rinsed in 20% ethanol, and transferred to a container andfully immersed in a phosphate-buffered storage solution comprising 0.25%glutaraldehyde, formaldehyde, ethanol, and Tween® (pH adjusted to 7.4with HCl and NaOH). Thereafter, the container is sealed and placed in anoven where it is heated to a terminal sterilization temperature of37.5±2.5° C. for 25 to 27 hours. The container is then cooled to roomtemperature and stored until the time of implantation.

Example 3 Treatment of Un-Fixed Tissue with Periodate Tissue Treatment

Bovine pericardial tissue (National Beef, Item #192769001, WO#58745266)was treated to mask antigens by the following procedure. Tissue wassoaked in a phosphate buffer containing 10 mM ethanolamine (Alfa Aesar,#36260) with pH 7.0±0.5 or 10 mM taurine with 7.0±0.5 pH (Sigma,#T0625). In both treatment groups, sodium periodate (Sigma, #311448) wasadded to yield a 20 mM solution with 7.0±0.5 pH. Tissue from the twogroups was incubated in one of three ways: 1) shaking at 4° C. for 18hours (New Brunswick Scientific, Innova 4230, refrigeratedincubator/shaker) 2) shaking at room temperature for 3 hours (VWR, Model1000, orbital shaker) and 3) shaking at 37° C. for 30 min. (VWR, Model1570, orbital shaker/incubator). After treatment the tissue was rinsedthoroughly in 0.9% saline (Baxter, #2F7124). The tissue was thenincubated in ethanolamine and sodium borohydride (Sigma, #452882) for 1hour at room temperature while shaking. Once again tissue was rinsedthoroughly in saline. One piece of tissue was placed in 10% NeutralBuffered Formalin (Lazer Scientific, NBF-4G) the remaining tissue wasfrozen in liquid nitrogen and stored at −80° C. for future analysis.

Histochemical Procedure

Tissue samples from each group were processed according to standardparaffin embedding procedure. Tissue was fixed overnight in neutralbuffered formalin. Tissue was then dehydrated through a series of gradedalcohol (Harleco, #65347); 70%, 80%, 95% and 100% and cleared in xylene(EMD Sciences, #XX0060-4) before being embedded in paraffin wax(McCormick Scientific, Para-Plast Plus #502004) using histology tissueprocessor (Sakura, Tissue-Tek VIP-1000). Each sample was then embeddedinto a wax block (Miles Scientific embedding station) and sectioned at˜5 μm using a rotary microtome (Reichert, HistoStat). The resultingslides (Fisher, #15-188-51) were heat-fixed overnight before staining.

Tissue from each slide was stained with standard H&E procedure andimmunohistochemistry, for the presence of a-galactose. Paraffin wasremoved by incubating in Xylene and rehydrated through a series ofgraded alcohol; 100%, 95% 80% and water. For H&E, slides were stainedwith Gill modified hematoxylin (Harleco, #65065), followed by stainingin Eosin Phloxine (ENG Scientific, #8923). After staining, slides weredehydrated and mounted (Fisher permount, # SP-15). Slides forimmunohistochemistry were incubated in isolectin-GS IB₄ conjugated toAlexa Fluor 488 (1:500, Invitrogen, 121411) in PBS, for 2 hrs at 37° C.

Results

Fresh, unfixed tissue was subjected to periodate treatments, with orwithout treatment according to the methods described herein (oxidizingagent such as periodate, capping agent, and reducing agent). FIGS. 2A-2Bshow immunohistochemistry for α-Gal expression following NexGentreatments of un-fixed tissues. FIG. 2A shows fresh, un-fixed tissuetreated according to the methods described herein. FIG. 2B shows fresh,un-fixed tissue treated with periodate only. The combined treatment offresh, unfixed tissues according to the methods described hereincompletely inhibits the binding of α-Gal antibody to the tissue comparedto control tissue treated with periodate only.

FIGS. 3A-3C show immunohistochemistry for α-Gal expression on un-fixedtissues treated with various types of periodate solutions. FIG. 3A showsun-fixed tissue treated with an in-house decell periodate. FIG. 3B showsun-fixed tissue treated with a Lifenet decell periodate. FIG. 3C showsun-fixed tissue treated with another commercial decell periodate.

Example 4 Treatment of Fixed Tissue with Periodate Tissue Treatment

Thermafix (tissue fixation followed by heat treatment; TFX) treatedpericardial tissue was obtained from isolation. Tissue was rinsed inthree changes of 0.9% saline (Baxter, #2F7124) before being soaked in aphosphate buffer containing 10 mM ethanolamine (Alfa Aesar, #36260) and20 mM sodium periodate (Sigma 311448) or 10 mM taurine (Sigma, #T0625)with 20 mM sodium periodate. Tissue from the both groups was incubatedat room temperature for 3 hours while shaking (VWR, Model 1000, orbitalshaker). After treatment the tissue was rinsed thoroughly in 0.9%saline. The tissue was then incubated in 0.06% ethanolamine and 0.25%sodium borohydride (Sigma, #452882) for 1 hour at room temperature whileshaking. Once again tissue was rinsed thoroughly in saline. Tissue fromeach group was stored in 0.625% glutaraldehyde (EW #400611) and theremaining tissue was incubated in 75% glycerol (JT Baker, #4043-01)/25%ethanol (EMD, #EX0276-3) for one hour at room temperature. Tissue wasthen laid out on absorbent pads to remove excess glycerol solution. Onepiece from each group was placed in 10% Neutral Buffered Formalin (LazerScientific, NBF-4G).

Histochemical Procedure

Tissue samples from each group were processed according to standardparaffin embedding procedure. Tissue was fixed overnight in neutralbuffered formalin. Tissue was then dehydrated through a series of gradedalcohol (Harleco, #65347); 70%, 80%, 95% and 100% and cleared in xylene(EMD Sciences, #XX0060-4) before being embedded in paraffin wax(McCormick Scientific, Para-Plast Plus #502004) using histology tissueprocessor (Sakura, Tissue-Tek VIP-1000). Each sample was then embeddedinto a wax block (Miles Scientific embedding station) and sectioned at˜5 μm using a rotary microtome (Reichert, HistoStat). The resultingslides (Fisher, #15-188-51) were heat-fixed overnight before staining.

Tissue from each slide was stained with standard H&E procedure andimmunohistochemistry, for the presence of a-galactose. Paraffin wasremoved by incubating in Xylene and rehydrated through a series ofgraded alcohol; 100%, 95% 80% and water. For H&E, slides were stainedwith Gill modified hematoxylin (Harleco, #65065), followed by stainingin Eosin Phloxine (ENG Scientific, #8923). After staining, slides weredehydrated and mounted (Fisher permount, # SP-15). Slides forimmunohistochemistry were incubated in solutions according to typicalimmunohistochemical staining with PBS (GBiosciences, #R028) rinses inbetween each step; 3% hydrogen peroxide (Sigma, #216763) in methanol(EMD, #MX0475P-1) for 15 minutes, 1% albumin, bovine serum (BSA, Sigma#A7030) in PBS with Tween® 20 (VWR, BDH4210) for 30 minutes,isolectin-GS 113₄ conjugated to biotin (1:2000, Invitrogen, 121414) inPBS for 1 hr at room temperature, Vectastain ABC reagent (VectorLaboratories, PK-1600) for 30 minutes and diamino-benzidine (DAB)reagent kit (KPL #54-10-00) for less than 3 minutes. Tissue wascounterstained with Hematoxylin (Harleco, #65065) for 1 minute anddehydrated in alcohol series before mounting in permount (Fisher,SP-15).

Results

Fixed tissue was subjected to TFX, with or without treatment withperiodate and/or capping with sodium borohydride and either ethanolamineor taurine.

FIGS. 4A-4F show that tissue fixed with glutaraldehyde has severeautofluorescence. The tissue shown was treated with TFX and periodate.Isolectin dye was used for staining (FIGS. 4A-4C).

TFX tissue was treated with formaldehyde bioburden reduction process(fBReP), then terminal liquid sterilization (TLS), and then stored inglutaraldehyde. FIGS. 5A-5B show α-Gal and DNA expression on fixedtissue treated with TFX only. FIG. 5C is a flow-diagram of the processused for this experiment, also described above. The presence of brownstaining demonstrates the inability of TFX treatment alone to block thebinding of α-Gal antibody to the fixed tissue.

TFX/fBReP tissue was subjected to periodate treatment followed bycapping and then stored in glutaraldehyde. FIGS. 6A-6D show the combinedtreatment of fixed tissue with TFX and periodate, a capping agent, and areducing agent. The upper panels (FIGS. 6A-6B) show tissue treated withethanolamine as the capping agent. The lower panels (FIGS. 6C-6D) showtissue treated with taurine as the capping agent. The absence of brownstaining demonstrates the ability of α-Gal antibody to bind the fixedtissue following the combined treatment.

TFX tissue was subjected to capping and ethanol/glycerol drying followedby ethylene oxide terminal gas sterilization. FIGS. 7A-7B show theresults of this treatment, and FIG. 7C is a flow-diagram of the processused for this experiment, also described above. The presence of stainingdemonstrates the inability of TFX treatment combined with capping toblock the binding of α-Gal antibody to the tissue.

TFX tissue was subjected to treatment with periodate, a capping agent, areducing agent, and drying. FIGS. 8A-8D show the results of fixed tissuehaving received such combined treatment. The upper panels (FIGS. 8A-8B)show tissue treated with ethanolamine as the capping agent. The lowerpanels (FIGS. 8C-8D) show tissue treated with taurine as the cappingagent. The absence of dark staining demonstrates the inability of α-Galantibody to bind the fixed tissue following the combined treatment.

Example 5 Comparative Analysis of Tissue Treatments

Relative levels of free α-Gal in variously treated tissues were comparedby an ELISA assay. Six tissue samples were treated by distinctcombinations of fixation/non-fixation; treatment according to themethods described herein (vic Diol oxidation, treatment with a cappingagent and treatment with a stabilizing agent); TFX treatment; capping,reduction and drying; and glutaraldehyde treatment alone. The six tissuetreatments compared are as follows: (1) unfixed bovine pericardium; (2)Treatment A: unfixed, bovine pericardium treated with a vic Dioloxidizing agent, a capping agent and a stabilizing agent; (3) TreatmentB: TFX-treated bovine pericardium; (4) Treatment C: bovine pericardiumtreated with a combination of TFX treatment and a vic Diol oxidizingagent, a capping agent and a stabilizing agent; (5) Treatment D: bovinepericardium treated with a capping agent, a reducing agent, and thendried; (6) Treatment E: bovine pericardium treated with a combinedtreatment of a vic Diol oxidizing agent, a capping agent and areducing/stabilizing agent and drying; and (7) glutaraldehyde-fixedprimate pericardium.

Following tissue treatment as specified above and in FIG. 9, each samplewas incubated with isolectin-B4, which is known to specifically bind toα-Gal. After overnight incubation, the isolectin-B4 remaining insolution was measured using a standard ELISA assay. Specfically, thetissue samples were cut into small pieces, frozen in liquid nitrogen andground into a powder. A solution of biotin conjugated, IB₄-isolectin(Invitrogen #I21414) and 1% BSA (Albumin, bovine serum; Sigma # A7030)were added to the ground tissue and incubated @ 37° C. overnight. Thesamples were then centrifuged to pellet tissue pieces to the bottom ofthe tube and the supernatant was transferred to a new tube. Samples werediluted before adding to the plate for IB₄-isolectin quantification.

As ELISA assay was performed using Isolectin-B4 in 1% BSA as a standard.Plates were coated with synthetic α-Gal-BSA (V-Labs, Ca_(t)#NGP1334) incarbonate buffer overnight at 4° C. The plate was washed three timeswith PBS containing Tween (0.01%) and then blocked in 1% BSA for 2 hoursat 37° C. A standard curve using Isolectin was added to the plate andthe diluted samples from above were added to the plate in triplicate.These were incubated for 1 hr at 37° C. The plate was washed 3 timeswith PBS-Tween. Vectastain substrate (Vector Labs, cat# PK-6100) wasadded to the plate and incubated for 30 minutes at room temperature. Theplate was washed 3 times with PBS-Tween and once with PBS only. ResidualPBS was carefully removed using an aspirator. Quantablu fluorescentsubstrate (Pierce, Cat#15169) was added and the plate was incubated for20 minutes. Stop solution was then added and the plate was read on aplate reader (Excitation: 320 nm, Emission: 420 nm).

The concentration of isolectin-B4 remaining in solution was used tocalculate the percent of total isolectin that is inhibited by thetreated tissue relative to a control. FIG. 9 shows the results of the invitro α-Gal ELISA assay for the variously treated tissues. Asdemonstrated in FIG. 9, the tissues treated with the methods describedherein exhibited a significant reduction in binding between α-Gal andisolectin-B4. These results indicate that the presently claimed tissuetreatment methods significantly reduce the quantity of free α-Galepitopes and thus reduce the antigenicity of treated tissues.

Example 6 Anti-α-Gal IGG Primate Study

A series of comparative analyses were conducting characterizing theanti-α-Gal IgG response in a group of five primates. Animal implantationwas performed at MPI Research. Five macaques were used for this study.Different combinations of test groups were implanted in the animals asdescribed below in order to see the immune response to tissue treatmentswith or without α-Gal. Six 6 mm tissue discs were implantedintramuscularly in the back of each animal. Three discs were implantedon one side and three discs were implanted on the other side. Bloodsamples (2 mL per time point) were collected before implant (baseline)and at 5, 10, 20, 45, 60, 75, 90, and 125 days after implant. The studywas terminated at 135 days. The blood was stored on dry ice and allowedto clot. Each sample was centrifuged and the serum was transferred to apre-labeled tube and stored in a −70° C. freezer.

The plate was coated with synthetic α-Gal-BSA (V-Labs, Cat# NGP1334) incarbonate buffer overnight at 4° C. The plate was washed three timeswith PBS containing Tween (0.01%) and then blocked in 1% BSA for 2 hoursat 37° C. The serum from different monkeys and different time points wasplated at different dilutions on the plate in duplicate. The serum wasincubated for one hour at 37° C. The plate was then washed 3 times inPBS-Tween. The secondary antibody, HRP conjugated, mouse anti-human IgG(Invitrogen, Cat#05-4220; 1:1000 in 1% BSA) was added to the plate andincubated for 1 hour at room temperature. The plate was washed 3 timeswith PBS-Tween and once with PBS only. The residual PBS was removed byaspirator and o-phenylenediamine dihydrochloride substrate (Sigma, Cat#P8806) was added and incubated for 20 minutes at room temperature. 3Msulfuric acid was added to stop the solution and the absorbance of theplate is read using a plate reader (@492 nm).

The first monkey received three glutaraldehyde-treated tissue samplesand three TFX-treated tissue samples. Both sample types produced ananti-α-Gal response in the monkey. Previous experiments havedemonstrated high calcification for glutaraldehyde and TFX-treatedtissues (data not shown). Monkeys two and three each received threecapped/reduced/dried tissue samples and three tissue samples treatedaccording to the method described herein. An anti-α-Gal response wasobserved. Previous experiments have demonstrated low calcification forboth of these types of treated tissue samples (data not shown). Thefourth monkey received four samples of tissue treated according to themethod described herein and two primate tissue samples. Neither thetreated tissue samples nor the control produced an anti-α-Gal response.The fifth monkey received six samples of primate pericardium as acontrol. The primate pericardium did not produce an anti-α-Gal response.

FIG. 10 shows the percent increase from baseline in the anti-α-Gal IgGresponse assay for each of the various tissue treatments as described indetail above. As demonstrated in FIG. 10, the presently claimed tissuetreatment significantly suppressed the anti-α-Gal response inxenographic tissue samples.

The invention being thus described, it will be obvious that the same canbe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed:
 1. A method for improving the performance of abioprosthetic implant, the method comprising: treating a bioprosthetictissue with an oxidizing agent that selectively oxidizes antigeniccarbohydrates having vicinal diols to produce free aldehyde or acidmoieties on the antigenic carbohydrate.
 2. The method of claim 1,further comprising fixing the bioprosthetic tissue with a fixation agentbefore treating the bioprosthetic tissue with the oxidizing agent. 3.The method of claim 2, wherein the fixation agent is selected from thegroup consisting of: an aldehyde, a formaldehyde, a dialdehyde, aglutaraldehyde, a polyaldehyde, a diisocyanate, a hexamethylenediisocyanate, a diacid, a diamine with a carbodiimide, a1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC), adipeoxy, and combinations thereof.
 4. The method of claim 1, furthercomprising treating the bioprosthetic tissue with a bioburden reductionsolution before treating the bioprosthetic tissue with the oxidizingagent.
 5. The method of claim 4, wherein the bioburden reductionsolution comprises formaldehyde.
 6. The method of claim 1, furthercomprising treating the bioprosthetic tissue with a capping agent aftertreating the bioprosthetic tissue with the oxidizing agent.
 7. Themethod of claim 6, wherein the capping agent blocks, removes or alters afunctional group that would have an adverse effect on the bioprosthesisproperties.
 8. The method of claim 7, wherein the capping agent is aprimary amine or an alcohol which combines with the aldehyde or acid toform an imine, amide or ester.
 9. The method of claim 7, wherein thecapping agent is selected from the group consisting of: an ethanolamine,a taurine, an amino sulfate, a dextran sulfate, a chrondroitin sulfate,a polyvinyl alcohol, a polyethyleneimine, an alpha-dicarbonyl, an aminoacid, a glycine, a lysine, an alkoxy alkyl amine, a 2-methoxyethylamine,an alkyl amine, a hydroxylamine, an aminoether, an amino sulfonate, anethylamine, a propylamine, a N-hydroxysuccinamide (NHS), aN-hydroxysulfosuccinamide (NHSS), a hydrazide, an oxirane, andcombinations thereof.
 10. The method of claim 1, further comprisingtreating the bioprosthetic tissue with a stabilizing agent.
 11. Themethod of claim 10, wherein the stabilizing agent reacts with the freealdehyde or acids or is a reducing agent.
 12. The method of claim 10,wherein the stabilizing agent is one or more selected from the groupconsisting of: a sodium borohydride, a sodium cyanoborohydride, alithium aluminum hydride, a carbodiimide, a1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), a pyridine, a2-chloro 1-methylpyridinium iodide (CMPI), and a Mukaiyama condensationreagent.
 13. The method of claim 1, wherein the oxidizing agent is aperiodate or acetate.
 14. The method of claim 13, wherein the periodateis a sodium periodate.
 15. The method of claim 1, further comprisingtreating the tissue with a secondary oxidizing agent to convert thealdehydes to acids.
 16. The method of claim 15, wherein the secondaryoxidizing agent is a sodium chlorite or hydrogen peroxide.
 17. Themethod of claim 16, further comprising capping the acids with a cappingagent to form an ester, the capping agent being selected from the groupconsisting of N-hydroxysuccinamide and N-hydroxysulfosuccinamide. 18.The method of claim 17, further comprising treating the tissue with acarbodiimide stabilizing agent to convert the ester to an amide.
 19. Themethod of claim 18, wherein the carbodiimide stabilizing agent is1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC).
 20. The method ofclaim 1, further comprising decellularizing the bioprosthetic tissuebefore treating the bioprosthetic tissue with the oxidizing agent.